The use of brakes and clutches in a wide variety of machines is well known. Among other things, brakes and clutches are both widely used in motor vehicles and railway vehicles. As detailed below, the operation of clutches and brakes is conceptually quite similar. One common feature is the use of friction to effect a connection in many clutches and most brakes. Both clutches and brakes operate on the principle of pressing relatively rotating surfaces into contact with one another. Typically, the relatively rotating surfaces have, or are caused to have, a high coefficient of friction.
Generally, the term "clutch" refers to a releasable coupling connecting the adjacent ends of two coaxial shafts for rotation together. A clutch is said to be "engaged" or "in" when the shafts are coupled and "disengaged" or "out" when they are released. Mechanical clutches fall into two main categories: positive engagement and progressive engagement. Positive engagement clutches are those which are either positively disengaged, so that no torque can be transmitted from the driving shaft to the driven shaft, or positively engaged by some mechanical devices such as splines, keys, or dogs. Progressive engagement clutches are clutches which are gradually engaged, so that the speed of the driving shaft falls and the speed of the driven shaft rises until both are rotating at equal speeds. The present invention is particularly related to friction type progressive engagement clutches.
In motor vehicles and other common applications, progressive engagement clutches generally operate on the principle of a flat friction surface being pressed against a relatively flat rotating surface. The simplest form of friction type clutch includes two opposed disks. In motor vehicles, one of the two disks is typically the engine flywheel. The other, generally less massive, is termed the presser or pressure plate. Other known forms of friction clutches include cone clutches and dry plate clutches. Presently, friction clutches are almost all of the dry plate type lined with a composite material having a high coefficient of friction. Most cars and commercial vehicles have single dry plate clutches. Some larger vehicles have double or triple-plate clutches.
As mentioned above, brakes, like clutches and other frictional engagement devices, operate on the principle of a friction surface contacting a relatively rotating surface. In brakes, the frictional contact is used to convert the kinetic energy of a moving vehicle into heat.
There are many known configurations of brakes. Most automobile brakes are friction brakes which generally fall into two categories: drum brakes and disk brakes. The typical drum brake is an expanding brake in which the brake shoes are brought into contact with the inside of the brake drum by means of an expanding mechanism. Disk brakes, on the other hand, normally use flat disks as the friction surfaces. There are, of course, many versions of these two general brake forms. A number of these versions employ pivoting friction shoes in which the brake pad is mounted on a support which is pivoted on an axis. An example of a pivoting shoe drum brake construction is the Rolls-Royce four shoe brake. An example of a pivoted shoe disk brake is the Girling swinging caliper brake. Additionally, pivoting shoe friction brakes are often used in railway vehicles.
As can be appreciated from the foregoing discussion, brakes and clutches operate on essentially the same principles. In fact, the earliest disk brakes were modeled on a multi-plate clutch. Today, of course, disk brakes typically have only a single disk and almost always have several sector-shaped friction pads of relatively small area. Thus, brakes and clutches differ from one another in that a clutch couples two shafts for rotation together whereas a brake couples a rotating shaft to a stationary member to stop rotation of the shaft. On the other hand, both brakes and clutches operate on the principle of a friction surface contacting a relatively rotating surface and that both generally use a friction pad or lining mounted on a support of some kind.
Most disk brake constructions, and some drum brake constructions, employ a brake pad support having a fixed orientation with respect to the surface the brake pad engages. The support is simply moved toward or away from an opposed surface to engage or disengage the brake; the orientation of the pad relative to the corresponding surface cannot be changed. This is typically the case with clutches as well.
In drum brakes, the pad, generally constructed of a friction material, is secured to a metal shoe. Typically, the linings are bonded to the shoes; it is also possible to secure the linings by riveting.
A complete disk brake assembly generally includes the disc, the caliper and the friction pads. The pads generally comprise a friction material pad supported by a steel back plate or shoe which is supported by, and moveable with respect to, a caliper. Early pads were made as individual pieces, which were usually bonded to the back plate. In some cases, spigots or rivets were used as a backup in case of bond failure. Presently, the friction material is typically integrally molded to a back plate provided with a number of openings which are filled by the friction material.
A relatively high performance friction material is typically used in both brakes and clutches. Two types of friction material are used in friction linings for brakes and clutches: woven and molded. Woven friction lining material is made by spinning fibers into a yarn, sometimes on a brass wire core. The fibers may be either natural or synthetic or a combination. For instance, asbestos (generally the white variety) and more recently, other fibers such as glass fiber, mineral woods, steel wool, and carbon fibers have all been used either singly or in combination. The yarn is woven into a cloth and then impregnated with a bonding agent. Molded type friction lining materials are typically made by mixing fibers and a resin or other bonding agent into a dough-like mixture and then molding the mixture under increased pressure and temperature conditions.
The friction material used in brakes and clutches tends to wear over prolonged use. This wear can be greatly accelerated by heat build-up. Accordingly, to reduce wear, or at least minimize its effect, in both brakes and clutches, it is desirable to obtain equal loading among the plurality of pads used and to obtain uniform temperature distribution across the entire surface of each pad. Temperature distribution depends, to some extent, on pressure distribution. However, perfect pressure distribution does not always lead to perfect temperature distribution. In particular, it has been found that even when equal pressure distribution can be achieved, "hot spots" develop on the engaged surfacer. These "hot spots" lead to quick wear.
The goals of equal loading and uniform pressure distribution are not always compatible. For example, one approach to equalization of the load among the pads is to pivotally mount the pad shoes so that, by pivoting, the shoes can equalize load. Although this solution is generally satisfactory for the purpose of load equalization, it causes uneven wear of the friction pad. In particular, since the pivot point of the shoe is spaced from the line of action of the friction force, a moment is generated by the frictional action of the pad on the surface it is pressed against. This moment causes pivoting or tipping such that, in the case of a brake, one end of the brake shoe friction pad, i.e., the "toe", moves into braking cooperation in advance of the remainder of the shoe when the brake equipment is in use. In this case, the toe portion of the shoe is worn away before the remainder of the friction pad. It can be easily appreciated that such uneven wear is disadvantageous because it makes it necessary to discard brake shoes which are worn only at one end. Also, the brake is less effective since the effective braking surface area is reduced.
For these reasons, in the design of pivoting shoe type brakes and clutches, a significant consideration is to locate the pivot pin as close to the center of pressure of the shoe face as possible to eliminate shoe tipping and uneven wear. For example, in U.S. Pat. No. 4,151,901, a circular vane and pin arrangement allows the brake pad to move about the center of mass to minimize shoe pivoting action. Unfortunately, this design includes rubber bushings which deteriorate with age and are subject to contamination. Further, the design is somewhat complex to manufacture. Also, the arrangement can only pivot about a single axis so that it can only provide alignment in two directions.
The present inventor has determined that the deficiencies of prior art pivoting shoe-type constructions generally result from the inability to locate the pivot point on the line of action of the frictional force, which would eliminate the moment generated by the friction force. Further, since the pad support shoe can only pivot about a single predetermined axis, the pad surface is usually not perfectly aligned with the rotating surface. Of course, as a practical matter, it is not possible to locate a pivot pin precisely at the friction surface, nor is it possible to pivot about more than one axis when a pivot pin is used.
In other known constructions, the pad is supported on a linearly moving piston type member. Unless the cylinder in which the piston moves is (and remains) precisely aligned with the surface contacted by the pad, this arrangement invariably results in uneven pressure distribution and/or wear.
Additionally, in high performance clutch and brake applications, it is important to obtain even temperature and substantially even pressure distribution across the entire face of the friction pad. Generally, because of manufacturing tolerances, it is difficult to achieve uniform pressure distribution. This can lead to excessive heat build-up and uneven wear. Consequently, the friction pads must be over designed to achieve an adequate factor of safety. Alternatively, the pads are simply designed for, and allowed to, wear unevenly. It should also be appreciated that uneven pressure distribution decreases the braking or clutching ability by reducing the braking or clutching surface area, thus requiring more power. To some extent, uneven pressure distribution results from the use of rigid pad supports in conjunction with load equalization and actuation devices, such as the calipers in disk brakes and the pivot/expander construction employed in drum brakes. Thus, the goals of equal loading and even pressure distribution are incompatible in these known constructions.
There are, of course, numerous designs for the calipers of disc brake assemblies. An important factor in the design of these calipers is the disc brake pad assembly retention. In some instances, the brake pads are retained in the caliper on a pivot pin or the like. This presents pressure distribution problems, as discussed above, because the pads can only pivot about a single axis, i.e., the axis of the pivot pin.
An important factor in the design of the caliper and the means for connecting the disc brake pads to the caliper is the prevention of squeal. The problem of squeal has been particularly troublesome in disc brakes and it is attributed not only to the brake lining material but to the caliper design itself. It has been found that offsetting the center of pressure on the friction pad by grinding a shallow step on the face of the piston results in a lessening of squeal. Another way of achieving a similar effect is to insert a specially shaped steel plate between the pad back plate and the piston face. In any case, it should be noted that there are applications in which the ability to adjust the center of pressure of the friction pad provides beneficial results such as reduction or elimination of squeal.
As noted above, this application relates to beam mounted support for the friction pads of brakes and clutches. It is believed that the concept of a beam mounted support has not, to date, been applied to brake and clutch pads. It is also believed that the most advanced work in the field of deflecting beam supports is that of the present inventor. For instance, the present inventor's European Patent Application (Publication No. 0343620) describes bearings having beam mounted bearing pads and methods of making the same. In this case, the bearings are supported by deflecting beam support structures to assist in the formation of a hydrodynamic wedge between a bearing pad and a rotating shaft.
Other patents have disclosed flexible support structures for supporting hydrodynamic bearing pads. For instance, U.S. Pat. No. 3,107,955 to Trumpler discloses a bearing having beam mounted bearing pads that displace with a pivoting or swing-type motion about a center located in front of the pad surface. The beam support is based only on a two dimensional model of pad deflection.
U.S. Pat. No. 4,496,251 to Ide, the present inventor, discloses a bearing pad mounted on web-like ligaments to deflect so that a wedge shaped film of lubricant is formed between the relatively moving parts.
U.S. Pat. No. 4,676,688, also to Ide, discloses a bearing construction which includes a plurality of discrete bearing pads supported in a carrier member. Each bearing pad includes a pad portion and a beam like support structure for supporting the pads as desired.
As mentioned above, the support structures described in these patents have not heretofore been applied to brakes and clutches.
Thus, there remains a need for an improved support for friction pads. More specifically, there is a need for a support which yields temperature and even pressure distribution and in some cases a need for an improved pivoting support.